The completion of wells involves a wide range of technological operations including the tapping of productive beds, formation testing in an open shaft, well lining and cementing, as well as the tapping of beds by perforation with subsequent outfitting of the well with undeground and ground equipment and bringing in of the well. The well productivity and duration of its exploitation depend upon the quality of execution of the afore-listed technological operations.
The principal objective in the completion of well is maintaining the natural filtration characteristic of the reservoir bed. A deterioration of the reservoir properties of the bed is caused by the penetration of filtrate and the solid phase of flushing fluid into the well face zone, which leads to the various irreversible processes in the bed. In so doing, there taken place the blocking of interstitial space with water-oil emulsion, the clogging of pore canals with the solid phase of flushing fluid, the plugging of interstitial space with swelling clay and products of chemical interaction of the flushing fluid filtrate with reservoir fluids and rock, reduction of the pore size due to rock deformation upon tapping, etc.
At present, the work related to the completion of wells is directed mainly towards:
setting up conditions for efficient execution of operations while tapping a bed for maintaining reservoir properties of the latter; and
intensifying the influx of fluid to a well by chemical, thermochemical, hydraulic and other means.
Investigations have shown that there are up to 3,000 outlet openings of pore canals per square inch of common sandstone (cf., Maly George P. Minimizing Formation Damage. Proper Fluid Selection Helps Avoid Damage, Oil and Gas J., 1976, 22/III, No. 74, No. 12, pp. 68-70). Said openings differ in shape and size and are readily contaminated with solid particles of inorganic and organic substances, and with polymers. Even if the majority of such canal openings in the shaft zone of the bed are contaminated, the well is still of some commercial value; however, its rating is extremely low and the flow rate decreases rapidly.
If the shaft zone of the bed is contaminated through a considerable depth, the initial permeability of clogged rock cannot be restored. However, in the case of contamination of but a part of the shaft zone of the bed (natural filter), the rock permeability is restored following an appropriate treatment of the well face zone.
Four main types of bed contamination are distinguished, namely:
1. The clogging of pore canals in the rock with solid particles penetrating the bed from the flushing fluid upon drilling, perforation and maintenance work in the well.
2. The contamination of the bed due to a reaction between clays contained in the bed with fresh water penetrating the bed serves a reason for the swelling or dispersion of the clays.
3. The contamination of the bed associated with an increased water saturation of rocks near the well shaft, due to fluid filtration into the bed upon drilling.
4. The formation of pockets in the bed and evacuation of loose rock from the well.
It has been found that one of the reasons for reservoir contamination is the formation of a zone of increased water saturation in the reservoir, the penetration of the interstitial space of the bed by clay particles from flushing fluid, the clogging by said particles of pore canals and hydration of clays, which results in the swelling or dispersion of particles in pore canals, injection of contaminated water into the bed and evacuation of sand upon destruction of poorly cemented rocks (cf., Izucheniye haraktera zagriazneniya plasta s pomoschiyu kernov i kernovyh analizov--Studying the Nature of Bed Contamination with the Aid of Cores and Core Analyses. Express Information, series on Petroleum and Gas Mining, No. 41, VINITI, Moscow, 1977).
In order to preclude the formation of a zone of increased water saturation in the shaft portion of the bed or to reduce the size of such zone, while tapping the bed use is made of inert flushing fluids, oil-based solutions, compressed gas, water. However, the substitution of, for example, water for flushing fluid in the course of bringing in of wells accomplishes nothing, inasmuch as the bed has already been clogged and no additional hydrodynamic or thermochemical effect will restore its initial rating, since productive reservoirs are made up mainly of hydrophilic materials and, upon interaction with clay particles activated with adsorptive-active reagents, exhibit stable assimilation in the pores (clogging).